Exposure to aeroallergens and air contaminants is hypothesized to be a major factor in the exacerbation of asthma. Results of our ongoing studies suggest that nitrogen dioxide (NO2) associated indoors with gas appliance use and outdoors with motor vehicle emissions, at levels well below the EPA Air Quality Standard, may be associated with respiratory symptoms in children at risk for developing asthma and with the exacerbation of asthma in asthmatic children. Our data are also suggestive of an interaction between indoor aeroallergens and NO2 concentrations in enhancing respiratory symptoms in infants and asthmatic children. The potential impact is great since 57% of homes in the US use natural or LP gas, traffic volume is increasing and virtually all homes have significant dust allergen levels. We propose a prospective epidemiologic study of 1,533 children with active asthma (5 to 10 years of age), to test the hypothesis that carefully quantified NO2 concentrations associated with indoor sources and vehicle traffic on state and interstate roads are associated with asthma severity. In addition, we will determine if home levels of NO2, in the presence of common indoor dust allergens (Der p 1, Der f 1, Bla g 1, Pel d 1, Can f 1 and fungi), are associated with an increased risk of asthma severity in children sensitized to those allergens. The above hypotheses will be evaluated while adjusting for factors known or suspected to increase risk of more severe asthma, including household and school characteristics. The study population will be drawn from elementary schools in 16 Connecticut towns using a modified version of the ISAAC questionnaire to identify children with active asthma. Initial risk factors will be assessed in a home interview, and asthma severity (symptoms, frequency of ER visits, medication use, lung function.) will be assessed prospectively for one year using telephone interviews administered quarterly. At the home visit, dust samples will be collected for allergen and fungi determinations and blood samples collected for antigen specific IgE determinations. Indoor and outdoor NO2 levels will be obtained quarterly for each home. Global Information Systems in combination with road vehicle density data will be used to assess the impact of traffic on asthma severity and outdoor NO2 concentrations at the home. Household gas appliances and traffic-related pollution may represent important environmental exposures for asthmatic children, for which effective interventions can be developed to reduce asthma morbidity.